Upgrades to clock module

The default clock rate is now set to 64th notes.  An additional (and
optional) parameter to the constructor is now the clock division, and
convenient constants for different clock divisions have been added to
defines.h.  Creating a clock that produces quarter notes running at
120BPM can be done like:

ModuleClock *clock = new ModuleClock(120, QUARTER_NOTE_CLOCK_DIVISION);

I also updated the SynthArp1 and SynthSubtractor to be compatible with
the new clock.